Idle speed control for a handheld power tool

ABSTRACT

Method for controlling fuel metering in a carburetor or a low pressure injection system of an internal combustion engine when the engine is operating at idle speed, the method includes the steps of:
         a) monitoring the engine speed;   b) determining a first variable (A) based on a first moving average algorithm using the monitored engine speed as input data;   c) determining a second variable (B) based on a second moving average algorithm using the monitored engine speed as input data, where the first moving average algorithm is arranged to react faster to an engine speed change than the second moving average algorithm;   d) comparing the second variable (B) to the first variable (A), where if 1) the second variable (B) is higher than the first variable (A): the fuel metering is set in a first leaner setting, and where if 2) the second variable (B) is lower than the first variable (A): the fuel metering is set in a second richer setting.

TECHNICAL FIELD

The present disclosure relates to an idling speed control method for anengine in which the fuel metering during idling is adjusted so as tofind an A/F ratio close to an optimal A/F ratio.

BACKGROUND OF THE INVENTION

In most engines for a power saw, a power cutter, a lawn mover andsimilar consumer products, the A/F ratio is manually controllable whenthe engine is idling, e.g. the electronic control system is only activewhen the engine is at working speed or above. It would therefore bedesirable to have a simple, non-expensive but efficient electroniccontrol method, without the need of adjusting the fuel or air supplymanually, when the engine is idling.

EP 0 715 686 B1 describes a method of controlling the engine A/F-ratio.Initially, the A/F-ratio is changed briefly. This could be effected forinstance by briefly throttling or stopping the fuel metering. Inconnection with the change, a number of engine revolution times aremeasured. The revolution times relate to engine rotational speeds chosenin such a manner that at least one revolution of the engine isunaffected by the change, preferably an engine rotational speed that issufficiently early for the A/F-ratio change not having had time toaffect the engine rotational speed. Further at least one forthcomingrevolution of the engine is chosen in such a manner that it is affectedby the brief A/F-ratio change. In this manner it becomes possible tocompute a revolution-time difference caused by an A/F-ratio change. Onthe basis of this revolution-time difference a change, if needed, of themixture ratio in the desired direction towards a leaner or richermixture is made. Thus using this method an optimal mixture can beachieved by testing how the engine reacts to a leaner or richer mixture.However this control is somewhat slow and mainly suitable forcontrolling the engine at working speeds.

PCT/SE06/000561 describes an idle speed control where the engine isstarted with a rich fuel setting and where the fuel setting is graduallymoved towards a leaner setting until an engine speed interval is reachedand if the engine speed comes above the engine speed interval the fuelsetting is gradually moved towards a richer setting. It also describes amethod for idle speed control using a single engine speed value wherethe fuel metering is decreased when the engine speed is below the enginespeed value and increased when the engine speed is larger than theengine speed value. This method will find a desired engine speed;however the A/F ratio may come far from an optimal A/F ratio.

U.S. Pat. No. 6,769,394 describes a method for controlling the fuelsupply to an internal combustion engine. An interval is allocated arounda desired parameter value, e.g. the engine speed. When the measuredparameter crosses the lower and/or upper threshold from below to abovethe fuel supply is cut off. And when the measured parameter crosses theupper and/or lower threshold from above to below fuel supply is switchedon. The method can be used at idle. This method will fluctuate around adesired engine speed; however the A/F ratio may come far from theoptimal A/F ratio.

EP 0 799 377 describes a method characterized primarily in that in thefuel supply system a fuel shut-off is effected during a part of theoperating cycle by means of an on/off valve shutting off the entire fuelflow or a part flow, and in that the shut-off is arranged to take placeto an essential extent during a part of the operating cycle when theintake passage is closed and consequently the feed of fuel is reduced orhas ceased. This means that the amount of fuel supplied can beprecision-adjusted by a slight displacement of one or both of the flanksof the on/off valve shut-off curve; this method will be referred to asPulse Width Modulation (PWM) of the fuel supply. However, EP 0 799 377also suggest that in particular for crank case scavenged two/four-strokeengines, the shut-offs can be performed every other, every third orpossibly every forth engine revolution instead upon each enginerevolution, in the case of a four-stroke engine, half as often. Ofcourse the on/off valve could also be set to be open every revolution.In that case a major fuel amount adjustment is made instead, forinstance by completely shutting off the fuel supply for a revolution.This can be done since the crank case in crank case scavenged two-strokeengines or crank case scavenged four-stroke engines can hold aconsiderable amount of fuel and consequently serve as a levellingreservoir, it is therefore not necessary to adjust the fuel supply foreach revolution when controlling the fuel supply to the engine, i.e.adjusting the fuel supply in one revolution will affect the subsequentrevolutions.

OBJECTS OF THE INVENTION

It is an object of the invention to provide a method for adjusting thefuel metering when the engine is operating at idle speed.

Another object of the invention is to provide a fuel metering duringidling which tunes towards an A/F ratio that is close to an optimal A/Fratio and preferably an A/F ratio that is slightly biased towards a richA/F ratio.

SUMMARY OF THE INVENTION

At least one of the above mentioned objects and/or problems are met byproviding a method for controlling the fuel metering in a carburetor ora low pressure injection system of an internal combustion engine whenthe engine is operating at idle speed. The method comprising the stepsof:

-   -   a) monitoring the engine speed;    -   b) determining a first variable based on a first moving average        algorithm using the monitored engine speed as input data;    -   c) determining a second variable based on a second moving        average algorithm using the monitored engine speed as input        data, where the first moving average algorithm is arranged to        react faster to an engine speed change than the second moving        average algorithm;    -   d) comparing the second variable to the first variable, where        if 1) the second variable is higher than the first variable: the        fuel metering is set in a first leaner setting, and where if 2)        the second variable is lower than the first variable: the fuel        metering is set in a second richer setting.

Preferably the first moving average algorithm addresses more weight to alower number of monitored engine speeds when determining the firstmoving average while when determining the second moving average moreweight is given to a higher number of monitored engine speeds, so thatthe first moving average algorithm is thereby arranged to react fasterto an engine speed change than the second moving average algorithm.

It is also preferred that when determining the second variable theoutcome from the second moving average algorithm is biased to correspondto a lower averaged engine speed for instance by subtracting the outcomewith a positive constant or multiplying with a factor smaller than 1.

According to another example when determining the first variable theoutcome from the first moving average algorithm is biased to correspondto an higher averaged engine speed for instance by adding the outcomewith a positive constant or multiplying with factor larger than 1.

Further according to an embodiment the first moving average algorithm isbased on a first plurality of samples of the monitored engine speed andthe second moving average algorithm is based on a second plurality ofsamples of the monitored engine speed, where the first pluralityincludes fewer samples than the second plurality. And where preferablythe first plurality of samples as well as the second plurality ofsamples are taken from the latest engine speed data of the monitoredengine speed.

In a further example the comparison of step d) is performed when thesecond variable is within an engine speed interval which is provided bya first engine speed threshold and a second engine speed threshold,where the second engine speed threshold is larger than the first enginespeed threshold. And where preferably if the second variable is higherthan the second engine speed threshold: the fuel metering is set in thesecond richer setting, and where if the second variable is lower thanthe first engine speed threshold: the fuel metering is set in the firstleaner setting.

According to one aspect of the invention the fuel metering is adjustedby means of a fuel valve, which fuel valve may e.g. be an on/off valveor a proportional valve. The fuel metering may also be adjusted by meansof an air bleed valve.

If the fuel valve is an on/off valve the richer setting and the leanersetting can be effectuated by means of corresponding fuel valve controlsequences determining which of the forthcoming engine revolutions theon/off valve is to be closed, during at least a portion of theircorresponding intake periods, respectively open, where the leanersetting includes more closings than the richer setting. For instance therich setting may corresponds to having the on/off valve fully opened andthe leaner setting to having the on/off valve closed during the intakeperiod of every second revolution.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will be described in the following incloser details by means of various embodiments thereof with reference tothe accompanying drawings, where

FIG. 1 is a schematically illustration of an internal combustion engineof two-stroke type in which the method according to the invention havebeen applied,

FIG. 2 illustrates schematically a carburetor of the internal combustionengine of FIG. 1,

FIG. 3 illustrates the engine idle speed control method according to theinvention,

FIG. 4 illustrates how the engine idling speed varies over theA/F-ratio,

FIG. 5 is a table showing a fuel shut-off schedule for the fuel controlof a crankcase scavenged engine 1, and

FIG. 6 is illustrates the difference by utilizing a fuel controlsequences according to FIG. 5 in contrast to a more rough regulation asdescribed in EP 0 799 377.

DETAILED DESCRIPTION OF THE INVENTION

The invention is particularly suitable for controlling a two stroke or afour stroke crank case scavenged internal combustion engine at idlespeed. The engine of FIG. 1 is known in the prior art and isincorporated in the description in order to clarify the invention. Inthe schematically illustrated drawing FIG. 1 numeral reference 1designates an internal combustion engine of a two-stroke type. It iscrank case scavenged, i.e. a mixture 40 of air 3 and fuel 4 from a fuelsupply system 8 is drawn to the engine crank house. From the crankhouse, the mixture is carried through one or several scavenging passages14 up to the engine combustion chamber 41. The chamber is provided witha spark plug igniting the compressed air-fuel mixture. Exhausts 42 exitthrough the exhaust port 43 and through a silencer 13. All thesefeatures are entirely conventional in an internal combustion engine andfor this reason will not be described herein in any closer detail. Theengine has a piston 6 which by means of a connecting rod 11 is attachedto a crank portion 12 equipped with a counter weight. In this manner thecrank shaft is turned around. In FIG. 1 a piston 6 assumes anintermediate position wherein flow is possible both through the intakeport 44, the exhaust port 43 and through the scavenging passage 14. Themouth of the intake passage 2 into the cylinder 5 is called intake port44. Thus the intake passage is closed by the piston 6. By opening andclosing the intake passage 2 varying flow speeds and pressures arecreated inside the passage. These variations largely affect the amountof fuel 4 supplied when the fuel supply system 8 is of carburetor type.

In FIG. 2 a conventional membrane carburetor is shown but also othertypes of carburetors that are arranged to supply fuel in a similarmanner for further treatment are possible. Supply of fuel 4 is affectedto fuel nipple 21 on the carburetor. From the fuel nipple 21 fuel iscarried to a fuel storage 22 which is delimited downwards by a membrane23. The fuel storage 22 and the membrane 23 operates as a fuel pumpdriven by the fluctuating pressure in the venturi 27 of the carburetor.From the storage 22 a line leads to a fuel valve 24 which connects thefuel storage 22 to the fuel lines 26, 25 leading to the venturi 27 inthe carburetor. The smaller channel 25 leads to the venturi 27,downstream the throttle valve 28, and is used as a so called idlingnozzle whereas the coarser channel 26 also leads to the venturi 27, butupstream the throttle valve 28, and is used as the principal nozzle.Because of the underpressure, which develops in the crankcase with theupward movement of the piston 6, fuel is drawn from both the idlingnozzle and the principal nozzle when the throttle valve 28 is open,whereas when the throttle valve 28 is closed fuel is drawn mainly fromthe idling nozzle. The fuel metering from the fuel storage 22 to theidling nozzle and principal nozzle is controlled by the fuel valve 24,thus by controlling the fuel valve 24 the fuel metering to the engine 1can be controlled. In particular the period when the intake port 44 isopen is of interest, since it is during this period the varying flowspeeds and pressures inside the intake passage 2 draws air and fuel tothe crank case. Thus having the fuel valve 24 closed as the intake port44 is open, in principal only air is supplied to the crank case. And,since the crank case in crank case scavenged engines can hold aconsiderable amount of fuel the crank case serves as a levellingreservoir. It is therefore not necessary to adjust the fuel meteringeach revolution, i.e. adjusting the fuel metering in one revolution willaffect subsequent revolutions. E.g. closing the fuel valve 24 everysecond revolution during the intake periods (i.e. when the intake port44 is open), corresponds to having a proportional valve half open eachrevolution. Consequently when using an on/off valve 24 in a crank casescavenged engine the fuel metering can be controlled by a)closing/opening the on/off valve 24 every second, every third, everyforth revolution and so on. It is also possible to operate the on/offvalve 24 according to b) a control scheme as described in relation toFIG. 5. Further it is also possible to control the fuel metering by c)opening and closing the on/off valve 24 during a portion of the intakeperiod, where the fuel metering is achieved by adjusting the timing ofthe opening and/or closing of the on/off valve 24 during the intakeperiod, the latter may be combined the fuel metering control of a) andb).

The fuel valve 24 may be any kind of on/off valves, i.e. a valve havingtwo positions opened and closed. However, the fuel valve 24 may also bea proportional valve. The fuel supply could also be controlled throughan air bleed valve controlling an amount of air bleed into a fuel supplyline to thereby adjust the amount of fuel delivered through the fuelsupply line.

The fuel valve 24 is preferably controlled by a control unit 9 whichreceives inputs from at least one sensor. An engine speed sensor(s) ESSprovides engine speed data to the engine, for instance the engine speedcould be measured as the time between two following ignition sparks.Further, the control unit 9 preferably receives inputs about theposition of the throttle valve from a throttle position sensor(s) TPS.The throttle position sensor(s) could for instance be a sensor thatdetects if a throttle trigger of a device comprising the en engine isactuated, i.e. the throttle position is not zero, or it could be asensor that detects if the engine is fully actuated, i.e. the throttleposition is full, or it could be a sensor(s) detecting both zerothrottle and full throttle or a more advanced sensor(s) detecting howmuch the throttle trigger is actuated. Needless to say other kinds ofthrottle position sensor(s) may also be used. Further, the control unit9 may of course receive inputs from other kinds of sensors than thosementioned above.

The idle speed control method described below can be implemented meansof a computer program in the control unit 9. For the control unit 9 todetermine if the engine is operating at idle speed, the control unit 9may use a wide variety of criterions. Such an idle criterion may bedifferent depending on the kind of sensor inputs available to thecontrol unit 9. For instance having a throttle position sensor onlydetecting full throttle, an idle criterion could be that full throttleis not detected and that the engine speed N is below a predeterminedengine speed (e.g. that an averaged engine speed is below a thresholdlonger than a predetermined time period). However, also otherconsiderations besides throttle position inputs and monitored enginespeed may be taken into account, for instance during a period afterstart of the engine, the fuel valve may be controlled according to adifferent method even though full throttle is not detected and theengine speed is below a threshold. Further, if the throttle positionsensor is able to detect zero throttle; an idle criterion could simplybe that the throttle position is zero. It should be realized that theidle speed control method described below can be used regardless of themethod on how to detect that the engine is operating at idle speed, i.e.the above mentioned examples of idle criterions is not intended to limitthe scope of the claims but should rather be seen as examples on how todetermine if the engine is operating at idle speed.

FIG. 4 illustrates in principle how the engine idling speed varies overthe Air-to-Fuel ratio. The left part of the diagram shows the enginehaving a rich mixture, i.e. the relative amount of fuel is comparablyhigh, and the right part of the diagram shows the engine having a leanmixture, i.e. the relative amount of fuel is comparably low. When theengine speed N has its peak N_(IDLE) _(—) _(MAX) the correspondingair-fuel mixture A/F_(IDLE) _(—) _(MAX) is said to be neither rich norlean; the engine has its optimum-power position. As can be seen in thediagram the engine speed declines faster on the lean side and for thatreason it is more desired to operate the engine during idle somewhat onthe rich side since the engine speed will be more stable and the riskfor undesired engine stops are reduced.

The idle control method which will be described below with reference toFIGS. 3 and 4 adjust the A/F-ratio towards the optimum-power position,slightly on the rich side thereof. In particular, the method is suitablefor idle speed control, but could also be used in other situations, e.g.when the engine is operating at start gas or at full throttle.

The method comprises the steps of a) monitoring the engine speedregularly providing new engine speed data as the engine runs, b)determining a first variable A based on a first moving average algorithmusing the monitored engine speed as input data; c) determining a secondvariable B based on a second moving average algorithm using themonitored engine speed as input data, where the first moving averagealgorithm is arranged to react faster to an engine speed change than thesecond moving average algorithm; and c) comparing the second variable Bto the first variable A, where if 1) the second variable B is higherthan the first variable A: the fuel metering is set in a first leanersetting, and where if 2) the second variable B is lower than the firstvariable A: the fuel metering is set in a second richer setting—thus thefuel metering will toggle between the second richer setting and thefirst leaner setting as long as the regulation is active as is indicatedby the pulse shaped wave in FIG. 3.

In step b) and c) it is preferred that the first moving averagealgorithm addresses more weight to a lower number of monitored enginespeeds when determining the first moving average while when determiningthe second moving average more weight is given to a higher number ofmonitored engine speeds. For instance the first variable A could becalculated through a first moving average over a first plurality ofsamples ×1 of the latest received engine speed data and the secondvariable B could be calculated through a second moving average over asecond plurality of samples ×2 of the latest received engine speed data,where the second plurality of samples ×2 are more than the firstplurality of samples ×1. For instance the first variable A could then becalculated as a moving average over the three last measured enginespeeds and the second variable B could e.g. be a moving average over theeight last measured engine speeds, i.e. A=(n1+n2+n3)/3 and B=(n1+n2+ . .. +n8)/8, where n1 is the last measured engine speed and n2 the secondlast and son on.

Preferably one or both of the variables A and B are biased so that theidle speed control is active at the rich side of the diagram in FIG. 4.This can be achieved by having the second variable B biased so as tocorrespond to an lower averaged engine speed, for instance bysubtracting the outcome from the moving average with a positive constantC1 or multiplying with factor F1 less than 1, e.g. B=(n1+n2+ . . .+n8)/8−C or B=F*(n1+n2+ . . . +n8)/8 and/or by having the first variableA biased so as to correspond to an higher averaged engine speed forinstance by adding the outcome from the moving average with a positiveconstant C2 or multiplying with factor F2 larger than 1, e.g.A=(n1+n2+n3)/3−C2 or A=F2*(n1+n2+n3)/3. The constants C2 or C1 could be0.5; i.e. corresponding to 0.5 rps (provided that the engine speed ismeasured in rps, i.e. in this example if rpm would be used C1 or C2would be 30). The larger the bias of A or B is, the richer thecorresponding A/F ratio that the idle speed control will adjust to willbe, i.e. an increased bias provides for a more safe engine operation butit will also consume more fuel. Therefore according to one example thebias is larger short after start when the engine is cold and decreaseswhen the engine has run warm.

The moving average algorithms for calculating the variables A and Bcould also be implemented by means of weighted moving averages, e.g.more weight could be addressed to the latest engine speed data. Forinstance A=(7*n1+5*n2+3*n3+n4)/16 and B=(n1+n2+n3+n4)/4−0.5, i.e. thefirst moving average algorithm addresses more weight to a lower numberof monitored engine speeds when determining the first moving averagewhile when determining the second moving average more weight is given toa higher number of monitored engine speeds, so that the first movingaverage algorithm is thereby arranged to react faster to an engine speedchange than the second moving average algorithm.

Through the comparison between these two moving averages A and B the A/Fratio will tune in to an A/F ratio slightly on the rich side of theoptimal A/F ratio, i.e. A/F_(IDLE) _(—) _(MAX).

In a further embodiment the regulation using the comparison between themoving averages A and B is active when the second variable B is withinan engine speed interval [y1, y2] which is provided by a first enginespeed threshold y1 and a second engine speed threshold y2, where y1<y2.Whereas if the second variable B is higher than the second engine speedthreshold y2: the fuel metering is set in the second richer setting tolower the engine speed, and where if the second variable B is lower thanthe first engine speed threshold y1: the fuel metering is set in a firstleaner setting to increase the engine speed. The first threshold mainlyserves to quickly adjust the fuel metering to an A/F ratio closer to thedesired whereas the second threshold y2 mainly serves as an upper limitfor the engine speed. Usually the upper threshold is above N_(IDLE) _(—)_(MAX) why the upper threshold will not be passed during the idle speedcontrol. However if for some reasons the engine speed curve is phaseshifted upwards (e.g. due to the conditions of the air filter or anyother reason) accordingly with the dotted lines in FIG. 4, the upperthreshold will serve as an upper limit of the engine speed andpreventing the A/F ratio to be leaner than A/F_(Y2). In any case theengine cannot run richer than the second richer setting and not leanerthan the first leaner setting, since these are the two extremes the fuelmetering is toggling between.

The engine idle speed control method described above requires that thefuel metering can be set in at least two distinct states, a secondricher setting and a first leaner setting. Below a number of examples onhow to adjust the fuel metering will be described as well as how to setin a rich or a lean setting.

Using a proportional fuel valve 24 the richer setting could e.g. befully (100%) opened while having the fuel valve partly open e.g. 30%open in the leaner setting. Of course, any other combination where thericher setting is a more open valve than the leaner setting is possible.

Using an on/off valve 24 the two states can be enabled by using PulseWidth Modulation as described above in relation to EP 0 799 377. E.g.one state could be enabled by having the fuel valve 24 fully openedduring the entire intake period while the other state could be enabledby having the fuel valve 24 closed during a portion of the intake periodor during the entire intake period.

Another way of providing different levels of the fuel metering whenusing an on/off valve 24 is by executing shut-offs every second, everythird, or every forth engine revolution, etc., and of course having noshut-offs. E.g. a richer setting could be implemented by having theon/off valve 24 open as long as the richer setting is active, i.e. noshut-offs, and the leaner setting by closing the on/off valve 24 everysecond revolution as long as the leaner setting is active, in thisexample the fuel metering would be toggling between 0% fuel reductionand 50% fuel reduction (as compared to the maximum fuel metering).

It is also possible to use a method where a shut-off schedule, as shownin FIG. 5, determines which positions the fuel is to be shut-off duringa forthcoming period of revolutions. A fuel valve control sequenceN_(S)/PL, where N_(S) is the number of fuel shut-offs during a periodand PL is the period length, determines which revolutions the fuel willbe shut-off during the period, by providing corresponding fuel shut-offpositions FC1, . . . , FCN. The leftmost row represents the fuel valvecontrol sequence 16/32. This means that the fuel supply is fullyshut-off for 16 revolutions of the 32 revolutions in the period, i.e. a50% fuel reduction in relation to a period utilizing the fuel valvecontrol sequence 0/32, which has no fuel shut-offs during the period.From the left hand of the table consecutive sequences increases from thefuel valve control sequence 16/32 till the rightmost fuel valve controlsequence 0/32, i.e. maximum fuel supply. Looking at the fuel valvecontrol sequence 7/32 it can be seen that the corresponding fuelshut-offs are scheduled to be affected at the fuel shut-off positionsFC1=1, FC2=6, FC3=10, FC4=15, FC5=19, FC6=24 and FC7=28. Thus the fuelsupply will be shut-off at seven evenly distributed revolutions duringthe period and providing a fuel supply of 78% of the maximum fuelsupply. Of course the fuel valve control sequence 16/32 corresponds tohaving the fuel valve closed every second revolution and the fuel valvecontrol sequence 0/32 corresponds to having the fuel valve fully openedfor every revolution during the period of revolutions.

An easy way to achieve evenly distributed shut-offs during a period ofrevolutions can be done by calculating the fuel shut-off positions as;FCn=(n−1)*(PL−N_(S))/N_(S)+n, for n=1 . . . N_(S), and rounding off theresult to nearest integer. And where PL is the period length and N_(S)is the number of shut-offs during the period. I.e. the fuel valvecontrol sequence Ns/PL provides corresponding fuel shut-off positions[FC1, FC2, . . . , FCN_(S)]. E.g. if the period length PL for example is64 and the fuel valve control sequence is 6/64, i.e. a 9% decrease offuel in relation to the maximum available fuel metering, the first fuelshut-off is done at the first revolution in the period, sinceFC1=(1−1)*(64−6)+1=1, the second fuel shut-off is done at the periodposition FC2=(2−1)*(64−6)/6+2=12, the third fuel shut-off is done atperiod position FC3=(3−1)*(64−6)/6+3=22, the forth fuel shut-off is doneat the period position FC4=(4−1)*(64−6)/6+4=33, the fifth fuel shut-offis done at the period position FC5=(5−1)*(64−6)/6+5=44 and the sixthfuel shut-off is done at the period position FC6=(6−1)*(64−6)/6+6=54.The table of FIG. 5 has been created using the above explainedalgorithm. Of course it should be realized that this particularalgorithm is merely an example on how the shut-offs can be evenlydistributed.

Using a shut-off schedule with the period length PL of 32 revolutions, arich setting could be e.g. the fuel valve control sequence 5/32, i.e.16% fuel reduction, and lean setting could e.g. be the fuel valvecontrol sequence 15/32, i.e. 47% fuel reduction. Of course, any otherpair of fuel valve control sequences where the richer setting providesfor a lesser fuel reduction than the leaner setting is possible. Furtherif the idle speed control method determines that it is suitable to shiftfrom the leaner setting to the richer setting or vice versa in themiddle of a period of revolutions, the current period can be stopped anda new period using a new scheme can be started.

FIG. 6 illustrates the difference by utilizing a fuel control sequencesas described in relation to FIG. 5, here however exemplified by a periodlength PL of 64 revolutions, i.e. 32/64, 31/64, . . . , 0/64 in contrastto shutting-off the fuel supply every second revolution, every third,every forth and so on as described in EP 0 799 377. As is evident fromthe figure the fuel valve control sequences 32/64, 31/64, . . . , 0/64provides for small and evenly sized fuel reduction steps, i.e. fuelsteps of 1/PL percentage units. However shutting-off the fuel supplyevery second revolution, every third revolution and so on; it can beseen that fuel reduction steps are far from evenly sized. The differencein fuel reduction between fuel shut-offs every second and every thirdrevolution is as high as 17 percentages units and between fuel shut-offsat every third and every fourth revolution, the difference is still ashigh as 8 percentages units.

Whereas the invention has been shown and described in connection withthe preferred embodiments thereof it will be understood that manymodifications, substitutions, and additions may be made which are withinthe intended broad scope of the following claims. From the foregoing, itcan be seen that the present invention accomplishes at least one of thestated objectives.

Even though the fuel supply system 8 has being described as being ofcarburetor type; the claimed method for controlling a fuel valve canalso be suitable in a low pressure fuel injection system.

The on/off valve 24 can for instance be a solenoid valve, anelectromagnetic valve, or a piezo valve.

Even though the engine have been shown with a crank case as a levellingreservoir, it would of course be possible to have other kinds oflevelling reservoirs for the fuel supply. For instance in a four strokeengine, instead of using a crank case a buffer volume anywheredownstream the fuel supply system 8 and upstream the intake valve(s) ofthe engine could be used.

Further if n1, n2, n3, n4, n5, n6, n7, . . . are the latest measuredengine speeds it would be possible to base the moving averages on asubset that to not include the absolute last measured engine speeds,e.g. the subset n3, n4, n5 could be used to calculate the first variableA.

1. A method for controlling fuel metering in a carburetor or a lowpressure injection system of an internal combustion engine when theengine is operating at idle speed, the method comprising the steps of:a) monitoring the engine speed; b) determining a first variable (A)based on a first moving average algorithm using the monitored enginespeed as input data; c) determining a second variable (B) based on asecond moving average algorithm using the monitored engine speed asinput data, where the first moving average algorithm is arranged toreact faster to an engine speed change than the second moving averagealgorithm; d) comparing the second variable (B) to the first variable(A), where if 1) the second variable (B) is higher than the firstvariable (A): the fuel metering is set in a first leaner setting, andwhere if 2) the second variable (B) is lower than the first variable(A): the fuel metering is set in a second richer setting.
 2. The methodaccording to claim 1, wherein the first moving average algorithmaddresses more weight to a lower number of monitored engine speeds whendetermining the first moving average while when determining the secondmoving average more weight is given to a higher number of monitoredengine speeds, so that the first moving average algorithm is therebyarranged to react faster to an engine speed change than the secondmoving average algorithm.
 3. The method according to claim 1, whereinwhen determining the second variable (B) the outcome from the secondmoving average algorithm is biased to correspond to a lower averagedengine speed for instance by subtracting the outcome with a positiveconstant or multiplying with a factor smaller than
 1. 4. The methodaccording to claim 1, wherein when determining the first variable (A)the outcome from the first moving average algorithm is biased tocorrespond to a higher averaged engine speed for instance by adding theoutcome with a positive constant or multiplying with a factor largerthan
 1. 5. The method according to claim 1, wherein the first movingaverage algorithm is based on a first plurality of samples (×1) of themonitored engine speed and the second moving average algorithm is basedon a second plurality of samples (33 2) of the monitored engine speed,where the first plurality includes fewer samples than the secondplurality.
 6. The method according to claim 5, wherein the firstplurality of samples (×b 1) as well as the second plurality of samples(×2) are taken from the latest engine speed data of the monitored enginespeed.
 7. Method according to claim 1, wherein the comparison of step d)is performed when the second variable (B) is within an engine speedinterval ([y1, y2]) which is provided by a first engine speed threshold(y1) and a second engine speed threshold (y2), where the second enginespeed threshold (y2) is larger than the first engine speed threshold(y1).
 8. The method according to claim 7, wherein if the second variable(B) is higher than the second engine speed threshold (y2): the fuelmetering is set in the second richer setting, and where if the secondvariable (B) is lower than the first engine speed threshold (y1): thefuel metering is set in the first leaner setting.
 9. The methodaccording to claim 1, wherein the fuel metering is adjusted by means ofa fuel valve (24).
 10. The method according to claim 9, wherein the fuelvalve (24) is an on/off valve having two valve positions an open and aclosed.
 11. The method according to claim 10, wherein the second richersetting and the first leaner setting of the on/off valve is effectuatedby means of a corresponding fuel valve control sequence determiningwhich of the forthcoming engine revolutions the on/off valve (24) is tobe closed respectively open, and where the leaner setting includes moreforthcoming closings of the on/off valve (24) than the richer setting,and where when closing the on/off valve the closing is effectuatedduring at least a portion of an intake period of the correspondingrevolution.
 12. The method according to claim 11, wherein the richersetting corresponds to having the on/off valve fully opened and theleaner setting having the on/off valve closed during the intake periodof every second revolution.
 13. Method according to claim 9, wherein thefuel valve is a proportional valve.
 14. Method according to claim 1wherein the fuel metering is adjusted by means of an air bleed valve.15. Method according to claim 1 wherein the engine is a crank casescavenged internal combustion engine.
 16. Method according to claim 1wherein the engine is a two stroke engine.